Caprolactam can be prepared by Beckmann rearrangement of cyclohexanone oxime. Such Beckmann rearrangement can be carried out by admixing cyclohexanone oxime to a reaction mixture comprising caprolactam, sulfuric acid and SO3. In such process the sulfuric acid and SO3 is a catalyst for the conversion of cyclohexanone oxime towards caprolactam. Such conversion is known to occur instantaneously.
Such a process is for example described in U.S. Pat. No. 3,914,217. In the process as described in U.S. Pat. No. 3,914,217 the Beckmann rearrangement is carried out in three stages in series. Cyclohexanone oxime is fed to each stage containing a circulating rearrangement mixture having a sulfuric acid+SO3 to caprolactam weight ratio and a SO3 content within certain ranges. The circulating rearrangement mixture of the first stage has a sulfuric acid+SO3 to caprolactam weight ratio of 1.33 to 1.80 (molar ratio of 1.55 to 2.17) and a SO3 content of 2 to 14 wt. %; the circulating rearrangement mixture of the second stage has a sulfuric acid+SO3 to caprolactam weight ratio of 1.14 to 1.31 (molar ratio of 1.32 to 1.55) and a S03 content of at least 0.82 wt. %, preferably 0.82 to 6.5 wt. % and the circulating rearrangement mixture of the third stage has a sulfuric acid+SO3 to caprolactam weight ratio of 1.00 to 1.13 (molar ratio of 1.15 to 1.33) and a SO3 content of at least 0.4 wt. %, preferably 0.4 to 4 wt. %. The reaction mixture obtained in the third rearrangement stage essentially containing caprolactam, sulfuric acid and optionally residual sulfur trioxide is sent to a reactor system together with ammonia, water and a solvent such as toluene. The sulfuric acid and SO3 are neutralized by converting the sulfuric acid and SO3 into ammonium sulfate and the caprolactam is simultaneously extracted from the ammonium sulfate solution formed in this system.
It is known that rearrangement can be effected at various values for the molar ratio M. This is in particular the case for the so-called more stage rearrangement in which the molar ratio M of the reaction mixture decreases in each further step.
However, it has been found that in the process as for example described in U.S. Pat. No. 3,914,217 the yield to caprolactam is still low when working at low molar ratio.
As used herein, the molar ratio M of the reaction mixture is defined as (nSO3+nH2SO4)/ncap, wherein nSO3=quantity of SO3 in reaction mixture, in mol (1 mol SO3 corresponds with 80 g), nH2SO4=quantity of H2SO4 in reaction mixture, in mol (1 mol H2SO4 corresponds with 98 g) and ncap=quantity of caprolactam in reaction mixture, in mol (1 mol caprolactam corresponds with 113 g). As used herein, with SO3 content (wt. %) is meant the amount of SO3 (g) relative to the total amount (g) of reaction mixture comprising sulfuric acid, SO3 and caprolactam. With SO3 is meant SO3 which can be analyzed as such in the reaction mixture.
The object of the invention is a process for preparing caprolactam by Beckmann rearrangement of cyclohexanone oxime with an improved yield to caprolactam when working at low molar ratio.
This object is achieved in that the SO3 content of the reaction mixture is between 9 and 20 wt. % and the molar ratio M of the reaction mixture defined as (nSO3+nH2SO4)/ncap is between 1 and 1.4, wherein    nSO3=quantity of SO3 in reaction mixture, in mol    nH2SO4=quantity of H2SO4 in reaction mixture, in mol    ncap=quantity of caprolactam in reaction mixture, in mol.
It has been found that with the process of the invention the yield of the rearrangement of cyclohexanone oxime to caprolactam is improved. Working at low molar ratio is advantageous as it results in the formation of less ammonium sulfate during subsequent neutralization.
In the process of the invention, by-products produced to a lesser extent resulting in an improved quality of the obtained caprolactam.
It has also surprisingly been found that, in spite of the high amounts of SO3, the caprolactam quality is not negatively influenced.
As used herein, the SO3 content (wt. %) is given relative to the weight of the reaction mixture comprising sulfuric acid, SO3 and caprolactam.